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(CNN) — White flags flying over the iconic Brooklyn Bridge where American flags should be — it happened this week in the dead of night, and no one seems to know what it means or who planned it.

“We’re looking across social media, Facebook, Twitter and other indicators to see if there are any claim of responsibility for this,” said John Miller, the New York Police Dept.’s Deputy Commissioner of Intelligence & Counterterrorism.

New details are emerging: a law enforcement official tells CNN DNA has been collected from tin pans, used to cover bridge lights Tuesday to hoist the flags without being seen.

On an Earthcam video, it’s hard to make out, but lights can be seen flickering around 3:30 a.m. near one flagpole.

But so far, no DNA match.

“At this time, it appears it has no particular nexus to terrorism or even politics,” Miller said. “This may be someone’s art project or it may be an attempt at making some kind of statement, but at this point, it’s not clear what that statement is.”

Or where the bleached 11-by-20 foot American flags with hand stitching came from. Officials are interviewing flag makers and checking internet sales.

Police are searching for four or five people captured in video during those overnight hours, and tracing five nicknames to see whether they belong to people who may be of interest.

Investigators are also systematically checking about 18,000 license plates of vehicles using the bridge that night.

And why was Monday night chosen? As officials say, a terror plot seems unlikely. If it was a prank, no one thinks it’s funny.